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163 Cards in this Set

  • Front
  • Back

What can transform the antennae into legs?

Nasobemia (Ant^ns) -a mutation studied by Rudie Turner from Indianna University.

What can transform a haltere into a wing?

bithorax, postbithorax

What does bicoid negatively regulate?

Caudal protein translation

What is bicoid required for?

specify anterior structures- gene function specific to higher insects.

What does Caudal do?

required to specify posterior structures in drosophila

What does Cdx stand for? Where is it expressed?

Caudal-type homeobox.


Caudal-type homeobox proteins are expressed at the posterior of the early embryo. They specify a posterior identity.

What are the gap genes?

expression of gap genes are controlled by maternal factors. They divide the embryo into broad domains. Gap genes respond to the bicoid gradient.

How do gap genes respond to bicoid gradients?

An increase in the concentration of bicoid protein in an anterior to posterior gradient results in an increase in the expression of hunchback gene in an anterior to posterior gradient.

How do gap genes regulate the expression of other gap genes?

Threshold concentrations predicted by the french flag model. "Different fates are specified by different concentrations of a morphogen."

Give an example of the regulation of gap genes by other gap genes with the french flag model or threshold concentrations.

Krupple gene activity is regulated by the concentration of the hunchback protein.


Krupple gene expression is repressed at high concentrations of hunchback.


Krupple gene expression activated at moderate concentration of hunchback.



Describe the bicoid protein as a morphogen.

concentration gradient:


mRNA and protein steepest at anterior


positional information:


gradient defines position along the anterior-posterior axis.


positional values:


cells interpret position based on thresholds of gradient.







Is bicoid a transcription or translation factor?

BOTH

Unknowns about bicoid protein

How is the bicoid gradient generated? asymmetrical localization of mRNA does not fully account for bicoid protein gradient.

What is the transcription factor heirarchy in anterior-posterior patterning of the drosophila body axis?

Maternal factors, gap genes, pair-rule genes, segmentation genes, selector genes.

Describe the function of the pair-rule genes.

Delimit the bounaries of the parasegment. Complex transcriptional control. Interpret the bicoid gradient and gap gene levels. Pattern defined just before cellularization.

What are the pair rule genes?

fushi tarazu (ftz) and even skipped (eve)

Describe how complex transcriptional control defines pair-rule transcription.

One control region control of eve controls the expression of eve gene in the third parasegment (or second eve stripe)




activators bicoid which controls the expression of the gap gene hunchback activate expression of eve.




Repressors define boundaries of the stripe, krupple, which is repressed by hunchback expression , and giant represses eve expression when cartain thresholds of the TF's are present.




Each eve stripe under the control of a unique control region that responds differently to the unique concentrations of gap gene transcription factors.

In the french flag model of pattern formation, what is a morphogen? (Give three properties)

-any substance that has a variable concentration


-different concentrations direct different fates


- concentration is constant at each end (source/sink)

Using the french flag model, how can "boundaries" of identity be formed?

Threshold concentrations- different fates are specified at different concentrations (ranges) of a morphogen.

What is the syncitium during early drosophila development?

multiple nuclei share a common cytoplasm. Somatic nuclei migrate to the embryo periphery.

What is a potential advantage for the formation of the syncitial blastoderm?

Precise coordination of 6000 nuclei. Cell-cell signalling not required during early pattern development (no cell membranes separating nuclei.)

1)Whatis the difference between parasegementsand segments during Drosophila development?

•parasegments areformed first, are a developmental intermediate•“outof register” compared to the segment boundaries•segmentsrepresent the final segementpattern in larvae and adults
2)Whatis the name of the group of proteins that are conserved across metazoaand regulate posterior development of the embryo?

Cdx (Caudal-type homeobox proteins)

What experiment showed that Bicoidis necessaryfor development of head structures in the fly?
•In bicoidmutants,where bicoid functionis lost. head structures do not develop
•4) Whatexperiment showed that Bicoid issufficientfordevelopment of head structures in the fly?
•if bicoidmRNAis transplanted back to the anterior end of a bicoidmutant,head structures will form in the anterior•If bicoidisectopicallyexpressed in the centre of an embryo, head structures will develop in that newlocation

General function of the pair-rule genes and the segmentation genes?

Pair rule genes- define the anterior boundaries of all 14 parasegments, but activity is temporary.


Segmentation genes: define anterior-posterior boundaries of parasegments and later segments.

After cellularization, segmentation genes respond to what and what happens?

pair- rule genes and position of parasegment boundaries become fixed.

Not all segementation genes are transcription factors. Give some examples

Wingless/Wnt and hedgehog are ligands in the signal transduction pathways.


-Allows cells to communicate once nuclei are confirmed to cells.

Describe the segmentation gene engrailed

A transcription factor that marks anterior boundary of parasegment- becomes the posterior compartment of the segment

What is a compartment?

Cell-lineage and/or cell fate is restricted by a boundary.




-cells are unable to move into adjecent compartments.


-expression of engrailed marks a compartment boundary.

Where is engrailed expressed

first expressed in a single line of cells at the anterior margin of the parasegment


-expression results of combinatorial regulation by pair-rule genes fushi tarazu and even skipped.

Compartments were first studied using what?

mitotic recombination by x-ray blasts. Through the formation of mutant clones.

When is engrailed expressed?

Throughout the life of the fly to maintain anterior-posterior compartments of segments

Animals that are heterozygous for a mutation (engrailed) can have recombination induced in what?

the imaginal discs through DNA damage of the cells.

What happens to engrailed mutants?

cells fail to respect the anterior-posterior boundary of the segment. Cells cross the A/P border in the wing.

what signalling forms a positive regulatory loop to maintain parasegment boundary and establish engrailed expression?


Wingless and hedghog

Describe wingless gene (Wnt ligand)

expressed in the posterior parasegment. Adjacent to cells that express hedghog. Wnt is a diffusible ligand with Frizzled as the receptor.

Describe hedgehog gene function

upregulates engrailed expression. Hedgehof (ligand) has patch as the receptor.

Describe the canonical Wnt signalling pathay

Wnt diffusible ligand binds frizzled the receptor. Dishevelled is the adaptor/co-receptor. APC/Axin/GSK-3B is for B-catenin destruction complex. TCF/LEF1 is a transcription factor.B-catenin is a transcriptional co-factor.

What happens to a wingless mutant

loss of segment boundary

Axin mutant? (Axin functions to limit Wnt overexpression)

too much Wnt signal. More anterior parasegment identity. Less denticles.

patched mutant?

spacing of segments closer together.

Describe planar cell polarity.

cells acquire a sense of directionality within groups of cells. A cell senses one end is different from the other, occurs in the plane of the tissue (i.e. the apical basal polarity of the epithelium.)

How are segment boundaries and polarity related?

Breaks in segment compartments-polarity of the segment is disrupted in that region, defect in local cell polarity at the breakpoint. Suggest that boundary formation initiated by Wnt and segment polarity are occuring at the same time of development.

Nobel prize for selector genes

Edward Lewis 1995

homeotic genes?

controls segment identity across anterior-posterior axis.

Gene components and where they are located in the homeotic selector gene complex

pattern head segments:


lab, pb, Dfd, Scr, Antp




pattern thoracic and abdominal segments:


Ubx,abd-A, abd-B

what are the homeotic selector genes?

Master regulatory genes, set fate of each segment. Control activity of genes that control differentiation of cells. Required throughout development to maintain pattern of gene expression.

Describe the homeotic transformtion mutation

mutation causes transformation of segment identity to another

bithorax mutant?

anterior haltere transforms to anterior wing

Hox genes?

one of the fundamental defining features of animal development. Highly conserved in anterior/posterior axis patterning in animals.

what are mutations that cause homeotic transformations?

Antennapedia and bithorax mutations

What are the two complexes of the homeotic selector gene complex?

Antennapedia(lab-Antp) and bithorax (Ubx,abd-A, abd-B)

co-linearity?

position of gene correlates to ant-post position of expression.

posterior dominance?

Hox genes expressed in more anterior segments (i.e. Ubx) are supressed in their ability to express segment identity by more posterior Hox genes.

What 3 genes are expressed in the parasegments in a combinatorial manner?

Ubx (5-12), abd-A(7-13) and Abd-B (10-14). Ground default state identity is parasegment 4

What do abd-A and Abd-B surpress?

Ubx expression

Describe the bithorax complex

•abd-A– identityof parasegments 7,8 & 9••Abd-B– identityof parasegments 10,11,12 & 13••Combinationof all three that defines identity••Lossof Ubx in bithorax mutant T3=T2 identity - anterior haltere (T3) → anterior wing (T2)

Describe the vertebrate body plan

vertebral column- segmented backbone surrounding the spinal cord


ant/post axis - head, trunk,tail. head enclosed in a skull.


dors/vent axis- spinal cord, mouth


Bilaterall symmetry- dorsal midline. gonads, lungs


left/right assymetry- heart and liver

All chordate embryos pass through what stage in development?

phylotypic stage when embryos are simliar in appearance. Common features: head, notocord, neural tube(earliest appearance of the nervous system) somites (blocks of mesoderm)

What is the pattern of development ?

Gametogenesis, fertilization, cleavage, gastrulation, notochord formation, neurulation, somitogenesis, organogenesis

Cleavage?

earliestcell division, rapid cell division through which embryo become divided intosmaller cells

Gastrulation?

dynamicprocess through which three germ layers (ectoderm, mesoderm and endoderm)organize themselves

Notochord formation

•columnof mesoderm located just ventral to neural tube

Neurulation

•formationof neural tube -precursor to nerve cord•

Somitogenesis

formation of regularly spaced blocks of mesoderm (somites) flanking notochord

Organogenesis

development of organs

Life cycle of Xenopus laevis

fertilization-cleavage to form blastula- gastrulation to form gastrula- neural tube formation (neurulation)- organogenesis-tailbud embryo- tadpole- metamorphosis

First cleavage of xenopus is along what axis

animal-vegetal axis

Blastula stage of xenopus is reached after how many divisions

12

descirbe the blastula of xenopus

radial symmetry. no visible signs of D/V or A/P axis. Marginal zone- equatorial ring around the embryo separating animal and vegetal regions. Blastocoel as a fluid filled cavity in animal region.

Where are the germ layers in the xenopus blastula?

Ectoderm- animal region.


Mesoderm- marginal zone (muscle, bone, blood, heart)


Endoderm- vegetal region

Gastrulation in xenopus

morphogenic process occuring in three dimensions where endoderm and mesoderm are internalized.

Blastopore in xenopus

small slit like infolding of the marginal zone on dorsal side, site of the embryonic organizer(spemann organizer) Organizes dorsal development and anterior-posterior axis.

mesoderm and endoderm enter the blastopore of xenopus and migrate via what to the future anterior of the embryo

involution.

Germ cells migrate as what?

coherant sheet of cells. First tissue to enter embryo are most anterior.

The ventral side of the gastrula initiates involution after what?

after the dorsal side. Delayed by similar cell movements occuring on ventral side.

ectoderm spreads to conver the embryo (gastrula) in a process called...

epiboly

What is the archenteron?

in the gastrula stage, a second cavity forms. It is the future gut cavity. blastocoel reduced in size

What spreads ventrally to cover the archenteron?

lateral mesoderm (mesoderm and endoderm spread to the left and right of the midline) spreads ventrally

xenopus laevis late gastula

blastopore close (future anus). The mesoderm contacts both the ectoderm and endoderm along its anterior-posterior axis.


Mesoderm is patterned along ant/post axis by the organizer


Ectoderm covers the embryo

Xenopus neurulation

dorsal mesoderm starts to develop into notochord (rod along dorsal midline) and somites.



Lateral mesoderm forms lateral plate mesoderm derived organs (heart and kidney)




edge of the neural plate forms neural folds which rise towards midline and fuse to form neural tube




The neural tube sinks below epidermis




Anterior neural tube becomes brain, middle and posterior neural tube becomes spinal cord.


What is neurulation?

formation of the neural tube.





What is the neural plate?

ectoderm located above the notochord and somites

What happens after neurulation?

tail bud stage occurs. Organogenesis initiates (eyes and ears start to develop.) three branchial arches form- form the jaws and bones of the face. Anterior neural tube divides. Dorsal part of the somites become dermatome. Ventral somite becomes vertebrae, trunk and limb muscles.

anterior neural tube is divided into what

forbrain midbrain and hindbrain

What does the lateral plate mesoderm form?

heart, kidney, gonads and gut muscles

ventral mesoderm?

blood forming tissues

endoderm

lining of the intestine, liver, pancreas lungs

During the tail bud stage, neural crest cells come fromt he edges of the neural folds after neural tube fusion and do what?

They detach and migrate as single cells between the mesodermal tissues to become sensory and autonomic nervous systems, pigment cells and some cartilage of the skull.

Radial symmetry in xenopus embryo is broken by what

establishment of the dorsal side gastrulation results in an embryo with a distinct anterior to posterior axis and dorsal to ventral axis.

animal-vegetal axis is under control of what

maternal factors deposited prior to fertilization.

mid-blastula transition

change from maternal to zygoticgene expression

What is Vg-1 (vegetalizing factor 1)

Transforming growth factor TGF-B family member. (ligand)

localized to vegetal cortex.


protein translated upon fertiliazation.


early signal for mesoderm induction.

XWnt-11

Wnt/wingless secreted signalling protein. mRNA localized to vegetal cytoplasm.


Reguired for establishment of dorsal-ventral axis.

Veg-T

Transcription facotr. T-box family of TF's/


mRNA localized to vegetal hemisphere.


required for endoderm and mesoderm specification.

Describe sperm entry for the establishment of dorsal side of embryo.

can occur anywhere in the animal hemisphere, breaks the radial symmetry of the egg. Future dorsal slide opposite to site of entry.

Cortical rotation.

The cortex (a gel like layer under the cell membrane) loosens from dense inner cytoplasm.




Rotates 30 degrees counter clockwise in a direction away from the site of sperm entry (caused by actin polymerization)




Movements of proteins and mRNA from vegetal region to site opposite to sperm entry.




Wnt pathway components (XWnt-11 mRNA and dishevelled protein, B-catenin are localized and help in patterning.

Describe the spemann organizer in Xenopus

located in the dorsal lip of the blastopore.


dorsal blastopore lip functions as a head organizer.


in the late gastrula embryo, the dorsal blastopore lip can only induce tail formation.


the properties of the spemann organizer change over time

Experiment of the spemann organizer in xenopus

The organizer taken form the early gastrula staged embryo and grafted to the ventral region of another early gastrula staged embryo results in a second embryo with dorsal structures as well as a complete head, trunk and tail.

During gastrulation, the anterior-posterior axis is laid down perpendicular to the dorsal-ventral axis with what mechanisms?

The first cells to enter the blastopore are dorsal mesoderm and endoderm and move towards the future head of the embryo.


The dorsal lip is not a uniform cell population, different cells occupy the lip at different times during gastrulation.

The dorsal and posterior region of xenopus are regulated by what.

Wnt signalling.

Describe the dorsal and posterior regulation

Nuclear b-Catenin has it's highest concentration on the future dorsal region of the embryo.


Dorsal/ventral axis and anterior/posterior patterning are separated by time.




In blastula, high Wnt signalling establishes the dorsal region.




In the gastrula, the longest duration of high Wnt signalling establishes the posterior region.

In xenopus, when is the D/V and A/P axis established?

D/V in the blastula


A/P in the gastrula

Before gastrulation, Wnt signal stabalizes what over time along the ventral/dorsal axis.

B-catenin

Functioning of the Nieuwkoop center is with the action of what combined factors to activate what?

B-catenin, maternal factors (VegT) on the dorsal side of teh vegetal pole activate siamois, a transcription factor.

Describe the Nieuwkoop center

First signalling center in the early blastula that develops in the dorsal-ventral region.


Sets the initial dorsal-ventral polarity in the blastula.


Spemann-mangold organizer is formed in late blastula, its formation and function relies on the earlier function of the Nieuwkoop center.

Dorsaling factors of xenopus

XWnt-11 and dishevelled.


Specify location of the dorsal side.


Necessary for formation of the spemann organizer and the nieuwkoop center.


Signal to Wnt signalling pathway and localized stabilization of B-catenin on dorsal side of embryo.

ventralizing factors of xenopus

cortical rotation failure. (Due to UV expsure of ventral side of embro)


Failure of translocation of dorsalizing factors blocks dorsal and anterior development


Deficient in structures formed on dorsal side and develop excessive amounts of ventral tissue (blood forming mesoderm)

experiments of the xenopus dorsalizaing factors

1) Transplantation of cell in 32 stage embryo from dorsal-vegetal region to ventral side.


-second dorsa/anterior axis.




2) injection of b-catenin mRNA into future ventral side of embryo.


-specify new nieuwkoop center, sufficient for formation of spemann o., second dorsal/anterior axis.




3) Xfz7 loss of function (Xenopus frizzled 7)


-necessary for dorsal structures




4) Treatment of embryos with LiCL


-dorsalizing of embryo at the expense of ventral structures


-lithium chloride inhibits GSK-3B


-GSK-3B part of B-catenin degradation complex

explant experiments from different regions ofthe blastula for assesment of mesoderm induction

removed parts of the embryo and cultured them in isolation.


Animal pole cells form ball of ectoderm


Vegetal pole cells form endoderm


Equatorial region (where animal and vegetal cells are adjacent) form ectoderm and mesoderm

What is an animal cap assay? What experiment used this for mesoderm induction determination and what was found?

remove animal pole from blastula embryo.


Animal cap cultured with vegetal region (marginal zone removed) -mesoderm induced




Mesoderm induced from prospective ectoderm in marginal zone in response to signals from endoderm.

Endoderm signal for mesoderm induction is what?

diffusible protein.



repeated animal cap cultures with vegetal region but seperate with a porous membrane showed what

only allows direct cell contact but small molecules such as proteins could diffuse through. So endoderm signal is a diffusible protein. The mesoderm was still induced.

Xenopus factors required for endoderm specification

maternal factor VegT specifies endoderm.




Sufficient-injection of VegT mRNA into animal cap-expression of endoderm-specific genes




necessary- block translation of VegT mRNA -endoderm does not form



Xenopus factors required for mesoderm specification

Xnrs (Xenopus nodal-related) factors




VegT required to activate expression of nodal related proteins in endoderm




nodal proteins involved in mesoderm induction in all vertebrates.


Family member of TGFB family of signalling



Xenopus factors required for ectoderm specification

Ectodermin. counteracts mesoderm inducing signals- limits mesoderm specification to marginal zone.




Fox1e- expressed in animal cells in late blastula


Fox1e absent- ectodermal cells mix with other germ layer and differentiate according to the new position.

what does the dorsal vegetal tissue contain?

nieuqkoop center-induces notochord and muscle

What does the ventral vegetal tissue do?

induces blood forming tissue, little muscle

animal caps are combined with different regions of the _______ for mesoderm patterning

endoderm. Diffenrent regions of the endoderm induce a different fate along the D/V axis

What 3 signals pattern the mesoderm along the D/V axis

1) the vegetal region produces a signal that induces the mesoderm and establishes the spemann organizer on the dorsal side




2) A distinct set of signals specifies the ventral mesoderm




3) The organizer releases a second signal that inhibits ventral fate- limits ventral fate to ventral side so that the mesoderm adjacent to the organizer can give rise to notochord and somites.

Describe the mesoderm and speman organizer induction process

TGF-B expressed in endoderm, induces mesoderm from ectoderm.


-xenopus nodal related proteins (Xnr) in all mesoderm and derrierre for trunk and tail mesoderm-major signals


-BMPs, Vg-1 and activin - minor signals

How do VegT and Wnt signalling activate nodal related proteins

VegT and B-catenin simulates Xnr hishest levels produced by nieuwkoop center- where VegT and B-catenin overlap.

Difference between dorsal and ventral mesoderm

expression of different nodal proteins at different concentration along the D/V axis

what is specified from the region where the nodal related signals are most intense for the longest period of time?

spemann organizer

TF's rquired for mesoderm development?

Brachyury (T-box family TF) and goosecoid

Describe brachyury

-earliest marker of mesoderm specification and patterning in all vertebrates


-first expressed in all mesoderm- later confined to notochord and tailbud


-necessary for development of posterior mesoderm



Describe goosecoid

-first gene expressed specifically in organizer in vertebrates.


-injection of goosecoid mRNA into ventral region mimics Spemann organizer transplantation- formation of secondary axis.

What is the threshold response to nodal-related proteins signalling pattern the mesoderm

activin is a diffusible protein that can activate the same receptors as nodal-related proteins




different levels of intensity of nodal related signal activate different genes in the mesoderm

What happens with activin soaked beads in animal cap assays

•Low concentration → only epidermal genes expressed (nomesoderm induction)


•Intermediate concentration → Brachyuryexpressed and muscle-specific genes


•Highest concentration → Goosecoidandorganizer induced as well as notochord

What factors ventralize the mesoderm

BMP-4 and XWnt-8. Organizer secretes signals that inhibit their activity.

BMP-4 antagonists?

Noggin, chordin, follistatin

XWnt-8 antagonists?

Frizzled-related protein (Frzb)

BMP-4 is initially uniform but is restricted by what

Ectodermin to marginal zone


-blocking BMP-4 activity embryo is dorsalized at the expense of ventral tissue.


-overexpression of BMP-4 ventralizes the embryo

Noggin

is strongly expressed by the Spemannorganizer-Prevents BMP-4 from binding receptor-expression of noggin protein rescues UV-ventralized embryos

Frzb

bindsto XWnt-8 and prevents it from activating receptors dorsally
Conserved Signalling Molecules andTranscription factors in Vertebrates
•mesodermdevelopment (Brachyury,Nodal, chordin, noggin)•organizerformation (Nodal, goosecoid)

what are the major deviations from amphibian development (chick and mouse)

epiblast- embryo is a layer of epithelium (sheet-like form) and is not a hollow blastula.


No distinct regions that correspond to 3 germ layers.


timing of specification of germ layers, epiblast has considerable cell proliferation and cell mixing. (before and after gastrulation)


primitive streak (like blastopore)


ventral closure



formation of what in chick embryo instead of ectoderm?

epiblast

primitive streak

equivalent of the blastopore, atgastrulation epiblast cells converge and ingress onthe primitive streak

ventral closure
gut cavity is formed by the folding ofthe lateral edges of the embryo to form a gut tube, tube is surrounded byendoderm, mesoderm then ectoderm

what is anologous to the amphibian blastula?

blastodisc

the blastoldisc or blastoderm arises through what?

cleavage

20 hours post-fertilization the chick blastodisc can be divided into two areas:

1) area pellucida- a translucent area on top of the cavity called the subgerminal space




2) surrounded by the area opaca (dark ring)

hypoblast

source of extra embryonic tissues, a layer on top of yolk

epiblast

embryo proper- develops from remaining upper layer of blastoderm

koller's sickle (crescent shaped ridge)

First visible structure that indicates anterior-posterior polarity of the embryo


Located at posterior end between area opaca and area pellucida


defines position of the primitive streak

posterior marginal zone

forms the junction of area pellucida and area opaca adjacent to sickle




defines dorsal side and posterior end of the embryo




region where gastrulation initiates

function of the primitive streak

•Laysdown the A/P axis

•Cellspass through the streak as individual cells, a process called ingression andspread out underneath the surface forming a bottom endoderm layer and middlemesoderm layer


•Cellsbecome specified to become mesoderm or endoderm during their passage throughthe streak. Remaining cells on surface become ectoderm


•Endodermeventually displaces the endoblast(extra-embryonic cells covering the yolk and form the hypoblast cells)

Hensen's node

•whenthe primitive streak reaches its greatest length, the anterior end begins toregress back to the posterior end

•theanterior end of the regressing streak is the Hensen’s node•condensationof cells that are also moving inwards


•major organizing center –similar to Spemann’s organizer

primitive streak vs the hensen's node

•theearlier primitive streak stage is when the germ layers are specified andinternalized

•theHensen’s nodeis an organizing centre – tissues are patterned behind it as is regresses


•anteriorstructures are patterned first

during the regression of the Henson's node, describe the mesoderm structures

cells first start to migrate anterior-wards along the midline (anterior to the primitive streak and Henson's node) under the epiblast to form the prechordal plate

what is the prechordal plate?

loose mass of cells anterior to head process

head process

anterior part of the notochord

more formation of mesoderm structures by the regression of Henson's node.

•Henson’snode moves toward the posterior – lays down notochordand the mesoderm on each side of

•anew somite is formed every 90 minutes (usedas a developmental clock)


•mesodermlateral to the somitesforms the lateralplate mesoderm (heart,kidneys, vascular system and blood)


•thenode eventually gives rise to stem cells that generate tailbud(post-anal tail)

ectoderm structures from the regression of henson's node?

•neural plate –ectoderm above notochord••theneuraltube formsabove the notochord ••headregion folds ventrally to generate head fold

•ventral closure -sides of embryo fold together to close midgut

Describe axis formation in the chick

•maternaldeterminants have little influence

•formationof the streak is highly regulative


•theblastodisc canbe cut up into many pieces and each will form a primitive streak


•posteriormarginal zone (PMZ) breaks radial symmetry of blastoderm•transplantationof the PMZ sufficient to initiate a new axiss

mesoderm induction and patterning in the chick

•mesoderminduction and patterning occurs in the primitive streak

•failureof streak elongation results in ventralizedembryo (blood cells and some muscle) •


•additionof activinrescues formation of dorsal structures (notochord)


•PMZanalogous to the Nieuwkoopcenter → transplantation will induce a second primitive streak

more mesoderm induction and patterning in the chick

•PMZ expression of Vg-1 and Wnt-8c→activatesNodal in primitive streak

•Nodal induces mesoderm and provide with adorsal patterning signal


•Hypoblast provides inhibitory signal forstreak formation


•Hypoblastreplaced by endoblast → allowsfor streak formation and Fibroblast Growth Factor (FGF) expression in koller’ssickle


•Nodaland FGF induce streak formation


•BMPexpressed outside the streak and inhibits formation of dorsal mesoderm


•BMPinhibited by chordin located at the tip (the most dorsal aspect) of the streak

similar organizing centers in xenopus and chich

PMZ → NieuwkoopcenterHensen’s node →Spemannorganizer

similar doral/ventral patterning in xenopus and chick

Anterior tip of the primitive streak→ most dorsal mesoderm (i.e. Hensen’snode, notochord then somites)






Posterior streak →lateralmesoderm (i.e. blood cells)

similar signals are patterning mesoderm in the chick and xenopus?

Wnt-8c and high Nodal →organizerand dorsal structures



Low Nodal (more ventral structures)




BMP ventralizing andinhibited by chordin

major differences in chick and xenopus mesoderm patterning

the organizer and ingressing mesoderm cells are moving as they are patterned along the d/v axis